Traveling wave tube



Feb. 23, 1960 A. ASHKIN 2,926,281

TRAVELING WAVE TUBE Filed May 31, 1956 2 Sheets-Sheet 1 FIG. 25

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A TTORNFV United States Patent 2,926,281 Y TRAVELING WAVE TUBE,

Arthur Ashkin, Far Hills, N.J., assignor to Bell TelephoneLaboratories,-Incorporated, New York, N.Y., a corporation of New YorkApplication May 31, 1956, Serial No. sss,49;s 19 Claims. c1. sis-3.6

This invention relates to apparatus which utilizes the phenomenon ofinteraction between space charge waves each of which istraveling along aseparate electron beam, for achieving amplification of the waves.

in the most common form of traveling wave tube operation, an electronbeam is projected along an extended path and awave is propagated along aspecially designed transmission line in close proximity to the beam.Interaction with the alternating electric field of the propagating wavecauses some electrons of the beam to accelerate and others to decelerateand so periodic bunchingof the beam occurs. Such bunching constitutes aspace charge wave on the beam, and continued interaction between thisspace charge wave and the wave propagating on the circuit results inamplification of the latter.

In the present invention it'is proposed to eliminate the necessity for aspecially designed transmission line for propagating a wave. Suchelimination is made possible since amplification in accordance with thisinvention is achieved, not by interaction between a propagating wave anda space charge wave on a beam, but rather, by interaction between twospace charge waves each one of which is on a different electron beam.Thus the second beam in effect takes the place of a transmission line.

, Interaction between two electron beams has been achieved hitherto inwhat have been referred toas double stream amplifiers. Intheseamplifiers two beamstravel at ditferentvelocities along extended pathsincloseprox- 'ice can be found in US. Patent 2,683,238, issued July 6,

1954, to S. Millman. Briefly, it can be said that in the charge waves onthe two beams and amplificationresults.

Such amplification is to be distinguished vir'ointhe amplif lengths. a

.A related object is to achieve amplification of the so-. called spatialharmonic type by use ofv the interaction between two electron beams. 1

To these ends a feature of the present invention is-a electron beams,each of which passes along an extended path, and the paths of the-twobeams'pass in coupling proximity with one another only at certaindiscrete intervals spaced apart in a succession along the beam lengths.For obtaining efficient amplification the coupling intervals areequispacedalong the beam lengths and the length of each is lessthan thelength of the spacing between successive intervals.

traveling wave tube having means for providing two" present inventionamplification is achieved by appropriately selecting the velocities ofthe two beams and the interval between adjacent coupling points so thatthe phase of the space charge waves on the two beams is the same atsuccessive coupling intervals. This does not require that the speed ofthe electrons of one beam travel at the same velocity as the'spacecharge wave on the other beam, butrather that while an electron of onebeam traverses the average distance between adjacent coupling intervals,the space charge wave on the other beam traverses substantially the samedistance plus an integral number of wavelengths. Hence interaction issaid to occur with the spatial harmonic components of the space chargewave.

In one embodiment of the present invention for use as a backward waveamplifier or backward wave oscillator, a first electron gun and targetelectrode are provided for projecting a first electron beam in apredetermined direction along an extended path. Additionally, a secondelectron gun and target are provided for projecting a second electronbeam in an opposite direction along a dilferentpath which is in couplingproximity with the first path only at a plurality of discrete equispacedintervals alongtheir lengths. When amplifier operation is desired, aninput transducer is required for launching a signal wave, to beamplified at the upstream end of one of the beams and an outputtransducer is required for extracting the amplified signal from thedownstream end of one of the beams. When oscillator operation isdesired, the input transducer isnot required. It may be eliminatedentirely or merely terminated suitably in its characteristic impedance.

In a second embodiment for use as a forward wave amplifier, the twobeams are projected along different paths which likewise are in couplingproximity only at certain discrete equispaced intervals along theirlengths, but they are directed in the same direction rather than inopposite directions. Input and output transducers are provided here asin the first embodiment.

The invention will beexplained in greater detail in the followingdescription taken in conjunction with the accompanying drawing, inwhich:

Fig, l is a longitudinal sectional view of a traveling wave tube whichemploys two oppositely directed elec-v tron beams in accordance with thepresent invention for forming a backward wave oscillator;

Fig.- 2 is a longitudinal sectional view of a traveling wavetube whichemploys two simi1arly' directed beams" for forming a forward waveamplifier; and

,- Figs. 3 to 5 are schematic representations of alternate techniquesfor projecting two electron beams which pass in coupling proximity toone another only at equispaced discrete intervals.

Referring now more particularly to the various figures of the drawing,Fig. 1 shows a traveling wave tube 10 for use as a backward waveoscillatorv comprising an evacuated envelope 11, typically of glass ornonmagnetic metal such as copper, enclosing two electron guns 12 and 13for forming two electron beams 14 and 15 and projecting those beamstoward co'lectors 16 and'17, respectively. Electron gun 12 includes acathode is, heater 195, beam forming electrode ill, and acceleratinganode 2 3; formed as part of envelope 11. The electron beam 14 emittedfrom this gun isv a solid cylindrical beam having substantially circularcross section. Electron gun. 13, inc Y eludes an annular cathode 22, aheater 23, an annular beam forming electrode 24, and accelerating anode25 formed as part of envelope 11. The aperture through anode 25 isannular for passing an annular beam 15 emitted by this gun. In practice,there is provided a solenoid or other magnetic means for establishing amagnetic field H whose flux lines extend along the length of the twobeams for focusing them.

A succession of cylindrical shielding members 26 are positioned tosurround beam 14 over a major portion of its length. These members serveto shield beam 14 from the surrounding annular beam 15 except atdiscrete intervals which lie between adjacent members. They thus act torestrict the points of interaction between the two beams to a successionof equispaced intervals. For most efiicient interaction. betweenrthebeams, the length of each of these intervals should be less than thelength of one of the cylindrical members. Moreover, since interactiontakes place between the two beams rather than between either or bothbeams and a wave propagating along the structure comprising thecylindrical members,

this structure is advantageously chosen to be nonpropagating.Alternatively, if a propagating structure is used, it should be chosento have a phase velocity nonsynchronous with both of the beam velocitiesto minimize coupling between the circuit and either beam.

In operation, wave manifestations existing as noise on one beam interactwith those on the other beam and amplification of certain components ofthe noise results. This amplification occurs between a spatial harmoniccomponent of the space charge wave on one beam and a spatial harmoniccomponent of the space charge wave ,on the other beam, and so will bereferred to as spatial harmonic amplification. By a proper choice of thevelocities of the two beams and the average spacing d between successiveintervals of interaction, such amplification can be obtained at anydesired frequency. It will be helpful for a full understanding of thepresent invention to derive the relationship between these parametersthat is necess'ary for amplification. Firstly, space charge waves atfrequency f and wavelength A will travel along one beam at a velocity v;given by:

At this velocity it will take time t to travel the distance d of Fig. 1,where t is given by:

In this time, in order to achieve spatial harmonic amplification, anelectron of the second beam must travel a distance n)\2-d, where n isany positive integer. This condition assures that the space charge waveson the two beams will have the same phase at the next interactioninterval and so amplification will be achieved. Since the time for onebeam to go a distance d at velocity v equals the time for the other beamto go a distance nA -d at velocity v;, we can equate these and obtain:

Substituting we obtain n 1 1 r (n 1).

From this equation we observe that spatial harmonic amplification can beobtained at any frequency f by appropriately selecting the spacing dbetween adjacent coupling intervals and the velocities v and v of thetwo beams.

Returning now to Fig. 1, traveling wave tube exhibits such spatialharmonic amplification of space charge waves on both beams along thesuccession of conductive cylinders 26. In operation beam 14 serves ineffect as a wave path for propagating a space charge wave from left toright in the tube and beam 15 as a wave path for propagating a spacecharge wave from right to left. Thus a closed regenerative loop isformed and oscillation occurs in this loop when the electron density ofthe beams is great enough to provide suificient gain for sustaining theoscillation. At that time both beams are characterized by low levelspace charge waves at their upstream ends (i.e., near their respectiveelectron guns) and, as a result of the harmonic amplification, highlevel space charge waves at their downstream ends (i.e., near theirrespective collectors). So an output signal is advantageously extractedfrom the downstream end of one of the beams.

In tube 10 an output signal is extracted from the downstream end ofannular beam 15 by a helical conductor 27. This helical conductor mustbe selected to have an axial phase velocity approximately equal to thevelocity of beam 15 for assuring strong coupling between the beam andthe helix. Additionally, when a helical conductor is used for extractingan output signal from the beam, it is important that the space chargemodulations on the beam are substantially symmetrical about the beamaxis to achieve strong coupling with the helix, as the fieldconfiguration characteristic of a wave propagating along the helix issubstantially symmetrical about the helix axis. Hence, with a helix asoutput transducer, it is important that conductive, cylinders 26 aresubstantially pitch'ess.

A conductive cylinder 28 is positioned within helix 27 to shield thehelix from the inner beam 14 so that the space charge wave on this beamdoes not interfere with the signal derived from the outer annular beam.Alternatively, the helix can be arranged to extract a signal from thedownstream end of the inner beam.

In practice, the various elements of the tube are maintained in place bysuitable support members which have been omitted from the drawing toavoid confusion. Additionally, there are provided lead-in wires fromsuitable voltage sources for maintaining the various elements atappropriate D.-C. potentials. In particular, each cathode is biased to apotential slightly negative with respect to the respective beam formingelectrodes and appreciably negative with respect to the acceleratinganodes so that the beam will be projected toward the respectivecollectors which are also biased positively with respect to the cathode.To maintain the velocities of the beams constant in their flow in theregion between accelerating anode 21 on the left and accelerating anode25 on the right, these anodes and all of the tube elements in the regionbetween them are maintained at the same D.-C. potential. However, as canbe noted from Equation 5, the frequency of operation of the oscillatorcan be varied by varying the velocity of either or both beams. To thisend variable voltage sources 31 and 32, shown schematically, areprovided for varying the voltage of cathodes 18 and 22, respectively. Inthis manner the velocities of the two beams, although constant withlength along the region of interaction, can be varied with time. Inpractice, however, the operating frequency of the tube of Fig. 1 willnormally be varied by varying the voltage from source 31 alone, andhence only the velocity of beam 14, as the velocity of beam 15 mustremain synchronous with the axial velocity of helix 27 for strongcoupling thereto. Such synchronism between the beam and helix can bemaintained while the velocities of both beams are varied if thevariation occurs only along the succession of cylinders 26 but not alongthe helix 28. This condition can be achieved by energizing helix 27 andcylinder 28 with a different D.-C. potential source than that used toenergize cylinders 26 and surrounding cylinder 29. With such anarrangement greater flexibility is obtained since the frequency ofoperation can be varied direction.

tion of the beam lengths.

by varying the velocity of either beam or of both beams.

Although tube has been described as a backward wave oscillator, it willreadily appear to one skilled in the art that it can be modified toserve as a backward wave amplifier. For use as an, amplifier the beamcurrent must be reduced to avoid undesired oscillation, and an inputtransducer must be provided. This transducer may be similar to the helixoutput transducer 27. It is, however, normally positioned at theupstream end of one of the beams for introducing at that end a signalwave to be amplified. In such a tube, as in oscillator tube 10, theoutput signal is extracted from the downstream end of either beam.

Fig. 2 shows a second illustrative embodiment of the present inventionfor use as a forward wave amplifier. This amplifier 11d differs from thebackward waveamplifier just discussed in that its two beams travel inthe same it comprises an evacuated envelope 111 cm closing two electronguns 112 and 113, which are similar to the guns described with referenceto Fig. 1;, for pro-v jecting beams 114 and 115 toward collectors 116and 117', respectively. A succession of cylindrical members 113 ispositioned to surround electron beam-114 over a major portion of itslength for obtaining spatial harmonic interaction between the two beamsover this region. As was discussed above, the length of each of thecylindrical members is preferably greater than the spacing betweensuccessive members. Also as discussed, the velocities of these two beamsand the average spacing d between successive cylindrical members 118 maybe chosen to achieve amplification at any desired frequency. Therelationship between these parameters for forward wave operation,derived in a manner similar to the derivation above, isthe following: 1

f d v v (6) where n is any positive integer, d is the spacing betweenadjacent coupling intervals, v is the velocity of the slower movingbeam, and v is the velocity of the faster moving beam.

In the operation of this tube, wave energy to be ampli fied isintroduced by coaxial line-121. it passes along helical conductor 122thereby initiating a space charge wave on the outer beam 115 which isprojected-at a velocity synchronous with the axial phase velocity of thehelix. This wave is amplified byspatial harmonic interaction with beam114 along the region of cylinders 113. An output signal is thenextracted from the amplified space charge wave on beam 115- by helicalconductor 123 whose axial phase velocity is chosen to be substantiallysynchronous with helix 122 and beam 115, and coupled by way of coaxialline 124 to an external circuit. Cylinders 126 and 127 are providedalong the regions of helices 122 and 123, respectively, to shield theinner beam from the signal wave on the respective helices. It is to beunderstood that the input wave may be employed to modulate the upstreamend of either beam and'the output signal extracted from the'downstreamend of either beam.

Figs. 3 to 5 show alternative arrangements for passing two beams alongpaths which are in coupling proximity only at discrete equispacedintervals along the beam engths.

In Fig. 3 two flat beams 311 and 312 are employed. These beams areprojected from two electron guns shown schematically and received by twocollectors. The beams are focused, along theirrespective lengths by amagnetic field H produced by a solenoid or other magnetic means (notshown). A succession of flat conductive members 313 is positionedbetween the beams along the major por- Spatial harmonic interactionoccurs between the beams at the various intervals between adjacentconductive members.

Fig. 4 shows an alternative technique, wherein one schematically,comprising the pole piece members 413 and 414 positioned at oppositeends of the device. An end view of this beam would appear as a circle.Beam 412, on the other hand, passes in a straight path parallel to theflux lines offield H. This path is chosen to intersect the path of beam411 at a succession of equispaced intervals. Spatial harmonicinteraction between these two beams occurs as a result of the couplingat these intervals.

Fig. 5 shows a further technique, wherein two beams 511 and 512 aredirected along the two singular equipotential surfaces characteristic ofa succession of spaced conductive rods 513 positioned between a pair ofparallel conductive plates 514 which are maintained at a potentialnegative with respect to the rods. Focusing of a beam along a singularequipotential surface in such an arrangement is often referred to asslalom focusing since the equipotential surface in side view resemblesthe path of a skier on a slalom. A discussion of this focusing techniquecan be found in United'States Patent No. 2,857,548, issued October 21,1958, of R. Kompfner and W. H. Yocoin. The beams 511 and 512 infollowing the equipotential surfaces intersect at a spaced succession ofintervals and spatial harmonic interaction occurs between the beams as aresult of the coupling at these intervals.

It should be understood that the beams of each of Figs.

3 to 5 can be oppositely directed as shown, for obtaining backward waveamplification or oscillation as discussed with reference to Fig. l, oralternatively, may be similarly directed for forward wave amplificationas discussed with reference to Fig. 2. In the former case the beamvelocities v and v and interval spacing d are chosen according toEquation 5 to obtain operation at a desired frequency, and in the lattercase the values of those parameters are chosen according to Equation 6.These equations must of course be modified slightly for application tothe arrangement of Fig. 4, since-the distance between adjacent couplingintervals is different along the two different beam paths of thatfigure. It should be further understood that the various. arrangementsof Figs. 3 to 5 are shown as schematic illustrations. In practice thesearrangements would be incorporated in tubes of the type shown in Figs. 1and 2 with suitable input and output transducers in energy transferrelationship with the various beams as discussed with reference to thosefigures.

The various embodiments discussed are merely illustrative of the generalprinciples of the present invention. In the light of this discussion,various other arrangements can be devised by one skilled in the artwithout departing from the spirit and scope of this invention.

What is claimed is:

1. A traveling wave tube comprising means for projecting a firstelectron beam along a predetermined extended path, means for projectinga second electron beam along a diflierent path which is in couplingrelation with the first beam only at a succession of substantiallyequispaced discrete intervals, the length of said discrete couplingintervals being shorter than. the spacing between adjacent intervals,means for varying the frequency of operation of said tube comprisingvariable voltage means for varying the velocity of at least one of saidbeams with respect to the other, and coupling means in energy transferrelation with one of said beams for extracting a signal wave therefrom,the other of said beams being shielded from said coupling means.

2. A traveling wave tube comprising means for projecting a firstelectron beam in a predetermined direction along an extended path, meansfor projecting a second electron beam in an opposite direction along apath which is in coupling relation with the first path only at asuccession of discrete intervals, the length of said discrete couplingintervals being shorter than the spacing between adjacent intervals,means for varying the frequency of operation of said tube comprisingvariable voltage means for varying the velocity of at least one of saidbeams with respect to the other, and coupling means in energy transferrelation with one of said beams at its downstream end for extracting asignal wave therefrom, the other of said beams being shielded from saidcoupling means.

3. A traveling wave tube comprising means for generating two separateelectron beams and for projecting each of said beams at a differentvelocity along an extended path, the path of the first beam passing incoupling relation with that of the second beam only at certainsubstantially equispaced discrete intervals, the length of said discreteintervals being shorter than the distance between adjacent intervals,the distance between corresponding points of adjacent coupling intervalsbeing approximately equal to where n is any integer, f is the meanfrequency of operation, v is the velocity of the slower moving beam andv; the velocity of the faster moving beam, means for varying thefrequency of operation of said tube comprising variable voltage meansfor varying the velocity of at least one of said beams with respect tothe other, an input transducer in energy coupling relation with one ofsaid beams at its upstream end for modulating said beam in accordancewith the variations in electrical strength of a signal wave to beamplified, and an output transducer in energy transfer relation with oneof said beams at its downstream end for extracting therefrom theamplified signal, the other of said beams being shielded from saidcoupling means.

4. A traveling wave tube comprising means for generating two separateelectron beams and for projecting each of said beams along an extendedpath, the two beams traveling in the same direction at differentvelocities and the path of the first beam passing in coupling relationwith that of the second beam only at a succession of substantiallyequispaced discrete intervals,.the length of said discrete intervalsbeing shorter than the distance between adjacent intervals, the distancebetween corresponding points of adjacent coupling intervals beingapproximately where n is any integer, f is the mean frequency ofoperation, v is the velocity of the slower moving beam and v thevelocity of the faster moving beam, means for varying the frequency ofoperation of said tube comprising variable voltage means for varying thevelocity of at least one of said beams with respect to the other, andcoupling means in energy transfer relation with one of said beams forextracting a signal wave therefrom, the other of said beams beingshielded from said coupling means.

5. A traveling wave tube comprising means for projecting a solidelectron beam along a first extended path and for projecting an annularbeam along a second extended path surrounding the path of the firstbeam, nonpropagating means positioned between said first and secondpaths for shielding said beams from one another except at a successionof substantially equispaced discrete intervals, at which intervals thebeams are in coupling relation, over a distance shorter than the lengthof said nonpropagating means, means for varying the frequency ofoperation of said tube comprising variable voltage means for varying thevelocity of at least one of said beams with respect to the other, andcoupling means in energy transfer relation with one of said beams forextracting a signal wage there;

from, the other of said beams being shielded from said coupling means.

6. A traveling wave tube comprising means for projecting a first beamalong an extended path, a plurality of cylinders spaced apart in asuccession surrounding said first beam and forming a nonpropagatingstructure, the length of each of said cylinders being greater than thespacing between adjacent cylinders, means for projecting a secondelectron beam along an annular path surrounding said succession ofcylinders and passing in energy coupling relation with the first beamonly at the intervals between adjacent cylinders of said succession,means for varying the frequency of operation of said tube comprisingvariable voltage means for varying the velocity of at least one of saidbeams with respect to the other, and an output transducer in energyexchange relation with one of said beams for extracting a signal wavetherefrom, the other of said beams being shielded from said couplingmeans.

7. A traveling wave tube comprising means for projecting a first beamalong an extended path, a plurality of substantially pitchlessconductive cylinders spaced apart in a succession surrounding said firstbeam and forming a nonpropagating structure, the length of each of saidcylinders being greater than the spacing between adjacent cylinders,means for projecting a second electron beam along an annular pathsurrounding said succession of cylinders and passing incoupling'relation with the first beam only at the intervals betweenadjacent cylinders of said succession, means for varying the frequencyof operation of said tube comprising variable voltage means for varyingthe velocity of at least one of said beams with respect to the other,and a helical conductor transmission line in energy coupling relationwith one of said beams for extracting a signal wave therefrom, the otherof said beams being shielded from said coupling means.

8. A backward wave oscillator comprising means for projecting a firstbeam in a predetermined direction along an extended path, a plurality ofsubstantially pitchless cylinders spaced apart in a successionsurrounding said beam and forming a nonpropagating structure, the lengthof each of said cylinders being greater than the spacing betweenadjacent cylinders, means for projecting a second electron beam in adirection opposite to that of the first beam along an annular pathsurrounding said succession of cylinders, the distance betweencorresponding points on adjacent cylinders being approximately equal to1 v2 where n is any integer, f is the mean frequency of operation, v isthe velocity of the first electron beam and v; is the velocity of thesecond electron beam, means for varying the frequency of oscillationcomprising variable voltage means for varying the velocity of at leastone of said beams with respect to the other, and a helical conductortransmission line having an axial phase velocity synchronous with thevelocity of one of said beams and positioned in energy transfer relationwith the down stream end of said one beam for extracting a signal wavetherefrom, the other of said beams being shielded from said couplingmeans.

9. A traveling wave tube comprising means for forming two separate flator ribbon-like electron beams and for projecting each of said beamsalong an extended path, the path of the first beam passing in couplingrelation with that of the second beam only at a succession ofsubstantially equispaced discrete intervals, the length of said discreteintervals being shorter than the spacing between adjacent intervals,means for varying the frequency of operation of said tube comprisingvariable voltage means for varying the velocity of at least one of saidbeams with respect to the other, and coupling means 9 in energy transferrelation with one of said beams for extracting a signal wave therefrom,the other of said beams being shielded from said coupling means.

10. A traveling wave tube comp-rising means for formingtwo separate fiator ribbon-likeelectron beams and for projecting each of said beams alongan extended path, a plurality of flat conductive members spacedrapart ina succession and positioned between said two beams whereby couplingbetween said beams is prevented except at intervals between adjacentconductive members, the length of each of saidfiat conductive membersbeing greater than the spacing between adjacent members, means forvarying the frequency of operation of said tube comprising-variablevoltage means for varying the velocity of at least one of said beamswith respect to the other, and coupling means in energy transferrelation 7 with one of said beams for extracting a signal wavetherefrom, the other of said beams being shielded from said couplingmeans.

11. A traveling wave tube comprising means for projecting a first flator ribbon-like electron beam along an.

extended path at predetermined velocity, means for projecting a secondflat or ribbon-like electron beam-along a different extended path at adifferent velocity, means forming a nonpropagating structure including aplurality of fiat conductive members spaced apart in a succession andpositioned between said two beams for preventing coupling between saidbeams except at intervals between adjacent conductive members, thelength of each of said flat conductive members being greater than thespacing between adjacent members, the distance between correspondingpoints of adjacent'ones of said coupling in-' tervals beingapproximately equal to where n is any integer, f is the mean frequencyof operation, v is the velocity of the slower moving beam and v is thevelocity of the faster moving beam, means for varying the frequency ofoperation of said tube comprising variable voltage means for varying thevelocity of at least one of said beams with respect to the other, andcoupling means in energy transfer relation with one of said beams forextracting a signal wave therefrom, the other of said beams beingshielded from said coupling means.

12. A traveling wave tube comprising means for generating a secondelectron beam and for projecting said beam along'a substantially helicalpath, means for generating a second electron, beam and for, projectingsaid second beam along a substantially straight path which substantiallyintersects the helical path at onepoint on each of its turns, thespacing between corresponding points on adjacent turns of the helicalpath being of greater length than the regions in which said pathsintersect, means for varying the frequency of operation of said tubecomprising variable voltage means for varying the velocity of at leastone of said beams with respect to they other, and coupling means inenergy-transfer relation with one of said beams for extracting a signalwave therefrom, the other of said beams being shielded from saidcoupling means.

13. A traveling wave tube comprising means for establishing alongitudinal magnetic field, means for projecting a first electron beamat an angle with the flux lines of said magnetic field whereby said beampasses along a helical path through said magnetic ,field, means forprojecting a second beam along a path substantially parallel to the fluxlines of said magnetic field, the path of the second beam substantiallyintersecting that of the.

said tube comprising variable voltage means for varying the velocity ofatleast one of said beams with respect to the other, and coupling meansin energy transfer relation with one of said beams for extracting'asignal wavetherefrom, the other of said beams being shielded from saidcoupling means.

14. A traveling wave tube comprising a nonpropagating succession ofrod-like conductive elements, means for maintaining said elements atanegative potential whereby there is established two substantiallysinusoidal equipotential paths through said succession, means forprojecting a first beam along one of said substantially sinusoidalequipotential paths, means for projecting a second beam along the secondof said substantially sinusoidal equipotential paths, said equipotentialpaths intersecting each other at a. succession of regions shorter inlength than the regions separating said succession of intersectingregions, and means for varying the frequency of operation of said tubecomprising variable voltage means for varying the velocity of atlea'stone of said beams with respect .to'the other, an output'transducer inenergy transfer relation with one of said beams for extracting a signalwave therefrom, the other of said beams being shielded from saidcoupling means.

15. A traveling wave tube comprising two substantially parallelconductive plates, a plurality of rod-like conductive elementspositioned between said conductive plates and spaced apart in asuccession substantially parallel to said plates, whereby there areformed two substantially sinusoidal equipotential paths through saidsuccession,;said equipotential paths intersecting each other at asuccession of regions shorter in length than the separation betweenadjacent intersecting regions, means for projecting a first beam in onedirection along one of said substantially sinusoidal equipotentialpaths, means for projecting a second beam in an opposite direction alongthe second of said substantially sinusoidal equipotential paths, meansfor varying the frequency of operation-of said tube comprising variablevoltage means for varying the velocity of at least one of said beamswith respect to the other, and an output transducer in energy transferrelation with one of said beams for extracting a signal wave therefrom,the other of said beams being shielded from said coupling means.

16. The combination of elements set forth in claim l5 wherein adjacentrod-like elements of the succession are spaced apart a distanceapproximately equal to Where n is any integer, f is the mean frequencyof operation, v isthe velocity of the first beam and v velocity of thesecond beam.

17. A traveling wave tube comprising means for generating two separateelectron beams, means for projecting each of said beams at a differentvelocity along is the an extended path, the path of the first beampassing in coupling relation with that of the second beam only at asuccession of substantially equispaced discrete intervals, the length ofsaid" discrete intervals being shorter than the distance betweenadjacent intervals, the distance between corresponding points ofadjacent coupling intervals being approximately equal to v of at leastone of said beams with respect to the other,

and coupling means in energy transfer relation with one of said beamsfor extracting a signal therefrom, the other of said beams beingshielded from said coupling means.

18. A traveling wave tube comprising means for projecting a firstelectron beam along a predetermined extended path, means for projectinga second electron beam along a different path, nonpropagating meanspositioned between said first and second paths for shielding said beamsfrom one another except at a succession of substantially equispaceddiscrete intervals, at which intervals the beams are in couplingrelation, over a distance shorter than the spacing between adjacentintervals, means for varying the frequency of operation of said tubecomprising variable voltage means for varying the velocity of at leastone of said beams with respect to the other, and coupling means inenergy transfer relation with one of said beams for extracting a signalwave therefrom, the other of said beams being shielded from saidcoupling means.

19. A traveling wave tube comprising means for projecting a firstelectron beam along a predetermined extended path, means for projectinga second electron beam along a different extended path which is incoupling relation with the first beam only at a succession ofsubstantially equispaced discrete intervals, a nonpropagating successionof members positioned between said first and second paths for shieldingsaid beams from one another except at said succession of equispaceddiscrete intervals, the length of said discrete intervals being shorterthan the spacing between adjacent intervals, means for varying thefrequency of operation of said tube comprising variable voltage meansfor varying the velocity of at least one of said beams with respect tothe other, and coupling means in energy transfer relation with one ofsaid beams for extracting a signal wave therefrom, the other of saidbeams being shielded from said coupling means.

References Cited in the file of this patent UNITED STATES PATENTS2,623,193 Bruck Dec. 23, 1952 2,652,513 Hollenberg Sept. 15, 19532,683,238 Millman July 6, 1954 2,694,159 Pierce Nov. 9, 1954 2,735,033Webber Feb. 14, 1956 2,757,311 Huber July 31, 1956 2,801,362 Hebenstreetet al. July 30, 1957 FOREIGN PATENTS 1,106,301 France July 20, 1955

